A Study On The Vertical Growth Mechanism Of Late Palaeozoic Contents Crust In The Eastern Precisional Margin

The Kalamaili granite belt is located in the eastern Junggar orogenic belt in northern Xinjiang.The orogenic belt is hosted between the Siberia plate and the Junggar plate.It is formed by subduction of the oceanic crust,closure of the oceanic basin and hyperplasia suture.The main body of the belt is composed of three deep faults(the Erqis-Mainebo fault,the Wulungu fault and the Kalamaili fault),three ophiolite belts(the Zhaheba ophiolite belt,the Almantai ophiolite belt and the Kalamaili ophiolite belt)and a large amount of granite formed in the post-collisional extension environment.The main research object of this paper is Kalamaili granite belt,which is mainly composed of Kamusite rock mass,Laoyaquan rock mass,Bailekuduke rock mass,Sabei rock mass,Huangyangshan rock mass and Sujiquan rock mass.The exposed form is NW-trending banded rock base,and the long axis extends about 99.17 km,with an area of 1072km².In this paper,a large number of data analysis is carried out for the above six granites,and it is found that there is a huge spatial difference in the long axis direction of the rock-based magmatism.The discussions are as follows:(1)Combined with the S-type Wutubulake rock mass in the adjacent area,the trace elements of the granite in the Karamaili rock belt are strongly depleted in Sr,Ba,Eu,Sc,enriched in Hf,Rb,Y,Zr,and the rare earth element curves are slightly left-leaning,showing a'V'shape,and strongly negative europium anomaly.It belongs to A₂-type granite,and shows that the formation conditions of the granite in the rock belt from west to east generally have a trend of evolution from reduction to oxidation environment,and the degree of magmatic differentiation is not obvious from high differentiation,and the gradual evolution of undifferentiation;(2)According to the zircon U-Pb age and Nd-Hf isotopic characteristics of each rock mass,the belt from west to east from Kamusite(324Ma?309Ma)-Laoyaquan(301Ma?300Ma)-Belekuduke(306Ma?283Ma)-Sabei(314Ma?290Ma)-Huangyangshan(305Ma?300Ma)-Sujiquan(304Ma)-Wutubulake(360Ma),the corresponding?Nd(t)range is 3.5?7.1,?Hf(t)range is 10.50?15.24,the granite?Nd(t)and?Hf(t)in the belt are positive,the overall difference between each rock mass is not large,suggesting that there is a new crustal growth in the region,and may have unique growth mechanism;(3)According to the analysis of zircon rare earth element data of granite in Kalamaili rock belt,it is concluded that the whole granite conforms to the characteristics of magmatic zircon,and there is a trend of sharp increase from La to Lu,that is,light and heavy rare earth fractionation is obvious,low LREE and high HREE,but La,Pr and Eu values are low,which shows a low valley in the spider diagram of rare earth elements.Comprehensive analysis shows that the characteristics of zircon rare earth elements indicate that the intrusion depth of Kalamaili rock zone belongs to the middle depth layer,and mainly comes from the upper crust and lower crust;(4)The whole-rock Nd isotopic data of 14 samples from the Karamari belt show that the?Nd(t)values of the granites are between+3.56 and+5.06,and the T_2DM values are between 521 and 646 Ma.The?Nd(t)values of the basic rocks(J-1 and J-2)are5.97(T_2DM=455Ma)and 4.45(T_2DM=573Ma).The above characteristics indicate that the granites in the Karamari belt originate from the depleted mantle,which may be formed by the remelting of new crustal materials or the underplating of basaltic magma;(5)Granites in the Kalamaili rock belt are classified into Nanling type,Zhejiang-Fujian type,Zhejiang-Fujian type,Nanling type,Himalayan type and Adak type in turn,and it is analyzed that the vertical growth of the newborn crust in the Kalamaili area is the thickest at the Kamusite rock mass,and the vertical growth of the newborn crust at the Laoyaquan and Beilekuduk rock mass is first thickened and reduced,and then it is thickened at the Huangyangshan rock mass,and finally the growth of the newborn crust at the Sujiquan and Wutubulake rock mass is gradually thickened and reduced;(6)Based on the geological phenomena reflected by the above data,the tectonic evolution model,magmatic genetic characteristics and the vertical growth mechanism of the crust in the Kalamaili rock belt are obtained,that is,the vertical growth trend of the newborn crust is:from the Kamusite rock mass to the Wutubulake rock mass,showing a trend that the vertical growth of the newborn crust gradually decreases.